Summary An automated acid-reaction kinetics tester (ARKT) was developed and used to evaluate various retarded-acid systems. Studies of several commercially available acid retarders revealed that some functioned by depositing hydrophobic films on carbonate surfaces while others were foaming agents that stabilized a CO2 foam barrier on the rock surface. In the absence of filter-cake development, viscosity imparted by the addition of gelling agents had little effect on reaction rate. Emulsification of acid was shown to reduce reaction rate, with the greatest retardation provided by oil-external acid emulsions.